ANY 16 colours can be used....but ONLY 16 colours.
BOTH images must use colours from the SAME palette.
ONLY the palette needs to have 16 colours.
The images can have less.
They CANNOT have more.
Once those rules are observed/adhered-to you will have NO colour issues.
Size is VGA. That means 'wall' must be 640x480.
Progress bar image must be 22x9.
That's about it, really.
Skinning doesn't get much simpler than this.
Remember ...if the wall image is NOT your own...provide a link to its source in the description with the submission so that permission to redistribute publicly can be checked.
One particular person has been spamming the uploads [a dozen a day] with NO info on sources...so is having them summarily rejected en-mass.....until he does....